Radial Mixing Devices for Rotating Inclined Reactors

ABSTRACT

Disclosed in this specification is the design for an internal mixing device which increases the plug flow like behaviour of the rotating inclined reactor.

PRIORITY AND CROSS REFERENCES

This patent application is a continuation of U.S. patent applicationSer. No. 13/456,340 filed 26 Apr. 2012 which is a continuation of U.S.patent application Ser. No. 12/524,776 filed 28 Jul. 2009 which is a 371National Phase entry from, and claims the benefit of priority of PCTApplication Serial No. PCT/EP2008/051406 filed 5 Feb. 2008.

BACKGROUND

The use of Rotary Kiln or cement kiln reactors for thermally treatingplastic pellets, or chips, in particular granules, pellets or chips ofcrystallizable polyesters containing at least 75% of their acid unitsderived from terephthalic acid, orthophthalic acid, 2,6 naphthalatedicarboxylic acid or their respective diesters has been describedpreviously in the patent application WO 2004/018541. An essentialfeature of WO 2004/018541 is the use of plug flow like behaviour toachieve uniformity of the properties of the granules at the exit. WhileWO 2004/018541 contemplates the use of baffles or internals it doesnothing to teach the design of such baffles necessary to increase themixing in the radial turning direction and yet maintain plug flow likebehaviour.

U.S. Pat. No. 3,767,601 describes a rotary kiln reactor for polyesterflake with internals for good mixing. U.S. Pat. No. 3,767,601 discloseskilns with internals for both batch and continuous processes. A batchprocess by definition cannot have plug flow, and nothing in U.S. Pat.No. 3,767,601 indicates that the internals are specially configured formixing while maintaining plug flow like behaviour.

SUMMARY

This specification discloses a horizontal rotating reactor having anaxis of rotation, wherein the axis of rotation is not parallel to thehorizontal line perpendicular to the force of gravity and wherein thehorizontal reactor has at least one mixing device wherein the mixingdevice has a height, width, and an equivalent length defined as thedistance between the plane perpendicular to the axis of rotation thatcontains the point where the mixing device first protrudes from the walland the plane perpendicular to the axis of rotation that contains thepoint where the mixing device stops protruding from the wall and theequivalent length of the mixing device is selected from the groupconsisting of equivalent lengths less than 1/10^(th) the length of thereactor. Further more preferred equivalent lengths of the mixing deviceare less than 1/12^(th) the length of the reactor, less than 1/14^(th)the length of the reactor, less than 1/15^(th) the length of thereactor, less than 1/16^(th) the length of the reactor, 1/18^(th) thelength of the reactor and less than 1/20^(th) the length of the reactor.It is further disclosed that there is more than one mixing device.

It is also disclosed that at least one of the mixing devices has holesto introduce a purge gas into the reactor. It is disclosed that there betwo or more mixing devices with holes. It is further disclosed that whenthere are two or more mixing devices with holes, that the mixing devicesare connected in a manner so that the purge gas can pass to from thefirst mixing device to the second mixing device through a connection.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the rotary reactor relative to the horizontalaxis and includes optional devices for the more commercial mode ofoperation.

FIG. 2 is the view of a plane perpendicular to the axis of rotation andincludes one mixing device, also known as a baffle or lifter.

FIG. 3 is a side view of a rotary reactor and shows the theoreticallength of the mixing device as measured relative to the axis ofrotation. Also shown is the length of the rotary reactor.

FIG. 4 is a side view of the reactor having a spiral mixing device withthe actual length of the mixing device being the distance measured alongthe mixing device as it spirals around the vessel.

FIG. 5 depicts one embodiment with several baffles in a planeperpendicular to the axis of rotation. It also shows the sample bedheight.

FIG. 6 depicts the embodiment where the vessel is not round, but itsouter wall rotates.

FIGS. 7 a and 7 b depict respective different ways to attach the mixingdevice to the wall of the reactor.

FIGS. 8 a and 8 b show the mixing device with holes to allow the purgegas to be introduced into the bed of granules or pellets.

FIG. 9 shows the various elements of a rotary reactor, with the bed ofmaterial shown in darkened area.

FIG. 10 shows the different types of flow patterns the solid phaseinside the reactor may experience as the speed of the reactor rotationincreases.

FIGS. 11 a and 11 b show respectively the Type 1 and Type 2 internalsused in the experiments.

DETAILED DESCRIPTION

Conventional wisdom is that rotational reactors with mixing devices(internals) have less plug flow behaviour or “degree of plug flow” thanthe same rotational reactor without mixing devices (internals). This isbecause it has been believed that the internals would create both axialand radial mixing components. The practitioner knows that axial mixingcomponent—causing the material to fall forward or backward along theaxis of rotation creates wide distributions of properties and is theprimary effect of any internals. This means that the degree of plug flowof a reactor with internals should always be less than that of the samereactor without internals.

Described and claimed herein is that discovery that when the equivalentlength of the internal mixing device is greater than about 1/10^(th) thelength of the horizontal reactor, the axial mixing component is greaterthan the reactor without a mixing device, but when the equivalent lengthof the mixing device is less than about 1/10^(th), in particular lessthan 1/20^(th), the length of the horizontal reactor, the degree of plugflow, as defined herein, increases, rather than decreases, as evidencedby the increased radial flow component, without a corresponding increasein axial flow component.

Therefore, described in this specification is the design of theinternals of a rotating reactor which allow for good radial mixing, yetmaintaining the plug flow like characteristic of the rotating reactor.The internal is referred to as a mixing device, or mixing devices ifmore than one is present. The mixing device is also known as a baffle orlifter. This internal is particularly useful in rotating reactors usedto increase the intrinsic viscosity of polyester resins.

In order to understand this description it is necessary to understandthe differences between plug flow and CSTR reactors, both from atheoretical basis and the real world applications where true plug flowand CSTR reactors do not exist.

First, all chemical reactors are characterized by a certain degree ofmixing. One of the two extreme cases, or end points, is the ContinuousStirred Tank Reactor (CSTR), which is the perfectly mixed system. A CSTRreactor is a reactor where 100% of the matter that constitutes thereactor hold-up has the same composition of the outlet stream. This isdue to the fact that the equipment theoretically performs as an idealcompletely mixed reactor.

On the other end of the reactor spectrum is the Plug Flow reactor, whichis the perfectly segregated reactor, namely a reactor whose hold-up canbe divided into an infinite number of hold-up slices with the specificcomposition each one different form the next one. The matter inside thereactor proceeds like a “plug” or like a “piston”

The practitioner knows that in reality, neither perfect CSTR nor perfectplug flow reactors exist and that the term “degree of plug flow” is usedto characterize equipment and reactors with respect to fluid dynamics ofinvolved gas, liquid and solid phases.

The degree of plug flow is expressed in the n-CSTR in series model,where n is the number of CSTR's which constitute the cascade or seriesof n-CSTR having the same distribution of the residence times as thereactor.

If n=1, then the reactor is the ideal perfect CSTR. If n=∞, then thereactor is the ideal perfect Plug flow reactor.

Vertical moving bed solid state polymerization (SSP) reactors (like theones currently used in present commercial solid state polymerizationtechnologies), generally speaking, have a degree of plug flow equivalentto a range of 4 to 8 CSTRs in series, and in any case lower than 10CSTRs in series.

It is known from literature that the highest degree of plug flowachievable on the solid phase inside a kiln type reactor is associatedwith “ROLLING” and “SLUMPING” flow regimes. FIG. 10 shows the type offlow regimes of the solid phase and the movement of the materialdescribed with the literature with slipping being associated with theslower reactor rotational speed and centrifugation associated with thehigher speeds.

The following degrees of plug flow were established using polyesterchips in the rotational reactors C1, C2 and C3, of the followingdimensions:

C1=2 meter long, 175 mm diameter (D) (without mixing devices): n=100 to150C2=22 m long and 2.1 m diameter (D) (without mixing devices): n=300 to400C3=(L>50 m; L/D=10 to 12 (without mixing devices): n<500

As is evident from the experimental data, a horizontal rotational SSPreactor without mixing devices is far more close to ideal plug flow thancurrent commercially available conventional vertical cylindrical movingbed SSP reactors.

Not all the reactor applications for chemical or polymerizationreactions require a plug flow behavior for the phases involved. However,it is necessary whenever the reaction kinetics is greater than the firstorder, since the high degree of segregation of the plug flow reactor,impeding the flattening of the reactants concentrations on the values ofthe same at the exit from the reactor, enhances the advancing of thereaction; furthermore, the plug flow behavior is necessary when a narrowdistribution of the properties of the finished product is requested.

The rotary reactor contemplated for these internal mixing devices is areactor which is part of a larger process used to continuously solidphase polymerize polyesters as described in WO 2004/018541, theteachings of which are incorporated in their entirety. This process isshown in various Figures accompanying this written description.

Polyester prepolymer granules stored in hopper 1-8, or other suchstorage device, are fed to a heater-crystallizer 1-6, where they areheated to a suitable temperature to cause the crystallization of thegranules, pellets, or chips with minimal sticking Preferably thereshould be no sticking. This crystallizer could be one of many in theart, however, the rapid crystallization at high temperatures ispreferred over slow crystallization and low temperatures.

Preferably, the crystallization step is carried out in a fluidised bedcrystalliser 1-6 by utilizing a gas flow rate sufficient to cause thepolyester granules to be fluidized with or without mechanical vibration.To this purpose inert gas or air can be used. Crystallization cangenerally be accomplished at residence times in the range of about 2 toabout 20 minutes and, preferably, from about 10 to about 15 minutes. Inthe case of polyethylene terephthalate resin, heating is achieved by usof a fluidizing medium (either air or inert gas) at temperatures in therange of about 140° C. to about 235° C. and preferably in the range ofabout 200° C. to about 225° C. The Residence time to crystallize thepolyester granules to the desired level depends on the crystallizationtemperature and crystallization rate of the polymer; low crystallizationtemperature requires longer crystallization time.

In general, polyethylene terephthalate prepolymer is crystallized to acrystallization degree corresponding to a density of at least about 1.37g/cm³. The polyester granules can also be crystallized by vaportreatment (see for example U.S. Pat. No. 4,644,049) or by high frequencyenergy field ranging from about 20 to about 300 MHz (see for exampleU.S. Pat. No. 4,254,253). After being crystallized, granules mayoptionally be fed into a preheater using purge inert gas. Thecrystallized polyester granules can optionally be dried after exitingthe crystallizer. However, drying it is not strictly necessary and it isless costly to polymerize “wet” polyester, as it is known from U.S. Pat.No. 3,718,621.

After crystallisation the polyester granules are solid-phasepolymerised. The crystallization and solid phase polymerization steps donot have to be conducted in strict temporal sequence, namely they havenot to be necessarily conducted one right after the other. One maycrystallize at one location and ship the crystallized materials toanother location to be solid phase polymerized.

The solid phase polymerization step is carried out in at least onehorizontal inclined (preferably cylindrical) reactor shown as 1-5 inFIG. 1. The reactor rotates around a central axis, 1-3, known as theaxis of rotation. This is similar to a “rotary kiln” The solid phasepolymerization reactor will be hereinafter for simplicity abbreviated as“HCIRR” and is referenced in FIG. 1 as 1-5.

An additional feature of the HCIRR reactor is the angle of inclination(α, in FIG. 1) which is the angle formed from the intersection of thehorizontal and the axis of rotation. As shown in FIG. 1 it is also theangle formed by a line 1-4 parallel to the axis of rotation 1-2.Preferred values for the angle of inclination are between 0.1° and 12°,more preferably between 1° and 6°, with a preferred maximum polyestergranule bed height of 4-5 meters. Advantageously, the combination of theinclination and the rotation, preferably with a speed between 0.1 and 10rpm of the HCIRR reactor (FIG. 1-5) and proper flow from one end to theother of the HCIRR reactor (FIG. 1-5), is provided and constant renewalof the inter-granular contact areas occurs so that polyester granules donot have a chance to creep into one another. As the weight itself of thegranules mass inside the reactor can not be ignored with respect toother forces acting as, for example the force of inertia, preferably thedesign and operating parameters of the reactor HCIRR 1-5 will be chosenso that the granules flow regime inside the reactor is characterized bya Froude Number Fr=(ω²×R/g) comprised in the range of 1×10⁻⁴ to 0.5;where ω is the angular velocity of the reactor; R is the internal radiusof the reactor and g is the gravity acceleration=9.806 m/s².

This flow regime, named “rolling”, is such that, when granules aresubmersed in the bed of solid, they behave as a rigid body and rotate atthe same rotational speed of the HCIRR reactor, and, when they come atthe surface of the solid bed, they slide on the surface itself. Thissolid flow regime facilitates having a true “plug flow like” behaviourof granules or pellets. Because absolute true plug flow is only atheoretical construct, the phrase plug flow like behaviour is used, withthe reactor exhibiting more or less plug flow like behaviour which meansthe reactor has more or less degrees of plug flow behaviour as describedpreviously.

The crystallized (or crystallized and preheated) polyester granules arepassed into the top of the HCIRR reactor (FIG. 1-5) (or in the firstHCIRR reactor of a series of HCIRR reactors, when the plant has morethan one HCIRR reactor in series) and pass through the HCIRR reactor (orthe reactors) due the force of gravity brought on by the reactor'sinclination as well as the reactor's rotation.

The granule flow rate through the HCIRR reactor 1-5 is controlled byregulating discharge from the HCIRR reactor itself. Such discharge isthen fed into a cooling device 1-7.

Polymerization is conducted in a stream of purge inert gas. Purge flowwell below the turbulent rate is generally preferred so to preventfluidization and entrainment of polyester granules. Furthermore, whenmore HCIRR reactors are present in series the inert gas flow rate willnormally be approximately equal. In the latter case, it is preferredthat the rate in each HCIRR reactor not exceed 1.25 times the rate inany other reactor in a reactor series. Preferably, furthermore, both inthe case of a single HCIRR reactor or a HCIRR reactor series the purgegas passes through the HCIRR reactor 1-5 counter-current to flowdirection of the polyester granules. Although also a purge gas flowco-current with the direction of the flow of the granules can be used,this latter configuration proves to be less efficient and generallyrequires a higher gas flow rate.

Suitable purge gases for use in the process of this invention preferablyinclude nitrogen, but also carbon dioxide, helium, argon, neon, krypton,xenon, air and certain industrial waste gases and combinations ormixtures thereof can be chosen.

Moreover, optionally, purge inert gas can be recycled to the reactor,after having been purified of organic impurities, preferably untilreaching a level of organic impurities less than or equal to 100 p.p.m.by weight (CH₄ equivalent).

In general the polymerization temperature will be included in the rangefrom just above the threshold polymerisation temperature to atemperature within a few (3) degrees Centigrade of the polymer stickingtemperature (which may be well below the melting point of the polymer).Usually this threshold temperature is 40° C. above the onset of thecrystallization temperature of the polymer.

For example, when polymerizing PET homopolymers and copolymers with upto 5% modification on a mole basis, a temperature in the HCIRR reactorwithin the range of about 170° C. to about 235° C. and preferably in therange of about 190° C. to about 225° C. is suitable. Temperatures in therange of about 205° C. to about 220° C. are preferred. These are alsothe suitable temperatures for the first HCIRR reactor in a series ifthere is more than one HCIRR reactor.

Modified PET copolymers containing from about 1 to about 3 mole percentisophthalic acid, a percentage based on total acids, are polymerized atabout 5 to 8° C. lower temperatures because their melt points are lowerthe PET homopolymers. Such copolyesters are less crystalline and have agreat tendency to stick at polymerization temperatures.

A central feature of the HCIRR reactor is that it revolves around anaxis of rotation (FIG. 1-3), wherein the axis is not parallel with thehorizontal, which is a line perpendicular to the gravitational pull. Theaxis of rotation is not parallel with the horizontal perpendicular togravitational pull when a material such as water or pellets are placedin a the higher end of the rotating reactor and when unaided by forcesother than gravity and the rotating reactor, move to the other, lowerend. The rotational axis is parallel to the horizontal lineperpendicular to gravitational pull when water, pellets, or anothersubstance placed in an end of the reactor and subjected to no forceother than gravity, (including friction forces), will not move to theother end. Obviously, this experiment is to be tried at both ends of thereactor, because if the axis of the reactor is not parallel to thehorizontal plane the material will move on one end but not move down thereactor on the other end. The angle formed by the intersection of theaxis of rotation and the horizontal line perpendicular to gravitationalpull is called the angle of inclination, (α).

While it is known in the art to attach mixing devices, such as bafflesor lifters, to the wall of the rotational reactor, however, the designfor plug flow like behaviour has not previously been disclosed.Disclosed in this specification are mixing devices which mix thematerial by removing the material at the wall and replacing it withmaterial towards the center of the bed of flow, but doing so in a mannerthat maintains or improves the plug flow like behaviour of the reactor.

It was directly observed during the experiments of rotational deviceswithout mixing devices and rotational reactors with mixing devices ofvarious lengths, that the granules/chips moved as a rigid body when theyare submerged and slide down (or “roll down”) from the upper side to thelower side of the chord that represents the upper boundary of the solidphase when the chips/granules were processed in a rotational inclinedreactor without a mixing device. The chord is the chord of the circledescribed by the cross sectional of the reactor wall wherein the chordis top of the granule bed as the granules are pulled up the wall of therotating vessel. This chord is depicted in FIG. 9. While rolling downthe chord, each chip is disturbed by the roughness of the bed surface(made up of other chips). The effect of this disturbance is that somechips were sent backward along the axis of rotation (up the reactor) andsome forward in the direction of reactor axis (down the reactor) thusgenerating a measurable and observable degree of axial dispersion. Whenthe equivalent length of the mixing devices was greater than 1/10^(th)the length of the reactor, there was more axial mixing than without themixing devices. Even if these reactors with mixing devices perform in avery high plug flow degree, they do not provide as high a degree of plugflow as the reactor without the internal mixing devices.

In one embodiment of the mixing device, at least one mixing device isattached to the wall of the reactor. FIG. 2 is cutaway view of thereactor in a plane perpendicular to the axis of rotation and containingone mixing device. The thickness of the mixing device is depicted as2-6. The reactor wall is depicted as 2-5 and the inner diameter thereofas 2-4. The mixing device protrudes from the wall at point 2-1, which isthe intersection of the line 2-9 which is the line tangent to the circlecircumscribed by the rotation of reactor wall at a point the furthestdistance from the axis of rotation 2-3. The height of the mixing devicein the plane view is the distance 2-8 which is measured from the pointof protrusion 2-1 to the top of the mixing device in the plane 2-2.

The thickness 2-6 of the mixing device in the plane perpendicular to theaxis of rotation is not essential to maintaining plug flow. However, thepractitioner will recognize that it must be strong enough to not deformor break under the stress due to the resistance of the solid phase whenrotating through the material it is trying to mix. Therefore, thethickness of the mixing device is determined by the required strengthwhich is determined in part by the material of construction and thetemperature of operation.

The height of the mixing device in the plane perpendicular to the axisof rotation is the distance from wall of the reactor to the top of themixing device. Although not essential to maintaining plug flow likebehaviour, it has been found that the best results are obtained when theheight is less than half the diameter of the circle defined by therotation of the point where the mixing device joins the wall of thereactor about the axis of rotation. (FIG. 2-3) For a circular reactor,the height of the mixing device would be less than one half the insidediameter, or the radius, of reactor. It is not necessary or requiredthat the height of the mixing device be constant along the length of themixing device. However, when height of the mixing device, also known asits radial penetration (protrusion) is less than 1/20th reactor diameterthe beneficial effect either on gas-chips renewal rate or on plug flowlike behaviour becomes negligible, even if length of mixing device isless 1/20^(th) total reactor length. At this height, such type ofinternals serve only as anti-slipping device, to avoid slipping of chipson the reactor wall.

There are two lengths of the mixing device. The physical length of themixing device is the length of the device measured from top of thedevice at the point where the mixing device first protrudes from thewall to the top of the device where the mixing device ends, or stopsprotruding from the wall. The point of protrusion at the beginning orend of the mixing device is when the height of the mixing device is lessthan about 1/20^(th) the diameter of the circumference of a circularreactor. The mixing device stops protruding from the wall when theheight is less than about 1/20^(th) the diameter of the circumferencedescribed by the rotation of the point of protrusion. The equivalentlength of the mixing device is the distance between the planeperpendicular to the axis of rotation that contains the point where themixing device first protrudes from the wall and the plane perpendicularto the axis of rotation that contains the point where the mixing devicestops protruding from the wall. This is depicted in FIG. 3.

FIG. 3, shows the reactor length of length 3-10 and the equivalentlength of two mixing devices, 3-11 and 3-13. Mixing device 3-13 is aspiral that traces the rotational curve of the reactor. The equivalentlength of device 3-13 is depicted as 3-12. which is the distance alongthe axis of rotation of the point of first protusion 3-1, whichcorresponds to 3-4 and the last protrusion, which corresponds to 3-6.The physical length of this mixing device is greater than the equivalentlength because the device traces the curve or spiral of the reactor.This is line 3-2. The second mixing device, 3-11, runs parallel to theaxis of rotation. Therefore its physical length and equivalent lengthare the same. The length of the reactor is not to scale, but is 3-10,which is the distance between point 3-8 and 3-9. Other referent pointson FIG. 3 are 3-5, the reactor wall, 3-3, the axis of rotation and 3-7,the diameter of the reactor.

This distinction between physical length and equivalent length is usedto describe a mixing device which protrudes from the wall, yet spiralsalong the reactor wall. The physical length could be three or fourcircumferences, yet still only have an equivalent length of 0.5circumferences. This is depicted in FIG. 4, where the mixing device, 4-1circles the reactor wall, 4-5, 2 times. This reactor has a diameter of4-2 and a length of 4-4. The equivalent length 4-11 is from the point4-6 to 4-10, as measured along the axis of rotation 4-3, while thephysical length is the measurement along the wall tracing the devicefrom 4-6 to 4-7 to 4-8 to 4-9 to 4-10. For a circular reactor and aperfect spiral, the physical length can be determined from Pythagorean'stheorem treating the circumference of the reactor as the base of thetriangle, the equivalent length as the height and the physical length asthe hypotenuse of the right triangle.

While the spiral configuration is possible, it is not believed that amixing device which completely circles the reactor at least onceprovides the desired mixing. However, such a mixing device iscontemplated. It is believed that better mixing is achieved when themixing device does not form a complete circle.

Forming a complete circle can be described as when the physical lengthof the mixing device is greater than the following formula, (Formula 1)

√{square root over ((a²+b²))}

Where a is the equivalent length of the mixing device and b is thecircumference of rotation which is the distance traveled in one rotationof the point of protrusion from the wall. It is also the innercircumference of a round annular reactor. It is believed preferredtherefore that the physical length be kept less than the value definedby Formula 1.

It is this equivalent length that determines whether plug flow likebehaviour is affected. What has been discovered is that when theequivalent length is less than about one-twentieth the length of thereactor, plug flow is enhanced. While equivalent lengths of less than1/20^(th) the length of the reactor are beneficial, it is also believedthat other lengths will work as well, therefore it can be said that theequivalent length be selected from the group consisting of equivalentlengths less than 1/10^(th) the length of the reactor, less than1/12^(th) the length of the reactor, less than 1/14^(th) the length ofthe reactor, less than 1/15^(th) the length of the reactor, less than1/16^(th) the length of the reactor, 1/18^(th) the length of the reactorand less than 1/20^(th) the length of the reactor.

The practitioner will recognize that there could be multiple mixingdevices in any given horizontal section of the reactor. For example,there could a first mixing device protruding from the wall at a firstpoint in the circumference of rotation and second mixing deviceprotruding from the wall at a second point 180° from the first point inthe circumference of rotation. Other mixing devices could protrude fromthe wall at points 90° and 270° from the first point. While the previousexamples try to balance the mixing devices, the number of mixing devicesand their location relative to the first mixing device is not essential.This configuration is depicted in FIG. 5.

FIG. 5 shows the reactor wall of 5-5, the axis of rotation 5-3, and fourmixing devices (5-1, 5-2, 5-4, 5-6), each 90° apart from each other.Also shown is the bed of material, 5-8, when the reactor is notrotating. The bed height is depicted as 5-10 and is the distance fromthe top of the bed 5-12 to the wall of the reactor at the thickest pointof the bed. 5-9 is the tangent line intersecting the point of protrusionand 5-7 is the inside diameter of the reactor.

If using multiple mixing devices, it is also not essential that themixing devices start to protrude and cease to protrude in the same planeperpendicular to the axis of rotation. They could be staggered along thelength of the reactor.

It is also contemplated that the mixing device be curved from the top ofthe mixing device to point of protrusion from the wall. This curve isalso relative to the plane intersecting axis of rotation and the pointof protrusion from the wall. The curve could arc into the direction ofrotation or away from the direction of rotation.

FIG. 6 shows an embodiment that could include the teachings of this art.In this embodiment, the actual reactor vessel (6-5) is not round, but inthis example, square. Even though the reactor 6-5 is square, itsrotation about the axis of rotation 6-3 will describe a circle 6-6 atits external corners and a further circle 6-8 at its points ofprotrusion for each baffle 6-1, 6-2, 6-4, and 6-7. It is this lattercircle which would be used to calculate the various dimensions for eachmixing device.

The mixing device can be attached to the wall in any manner. FIGS. 7 aand 7 b show a bolted and welded mixing device. In the FIG. 7 a, mixingdevice 7-2 is attached used in the bolt 7-1. In the FIG. 7 b, the mixingdevice 7-2 is welded to the wall and 7-4 shows a bead from a weld.

The mixing device should also be devoid of holes transversing the axisof rotation which are larger than the size of the granules, pellets,flakes or chips to be processed. In one embodiment holes arespecifically contemplated. Since the reactor of performs better when agas is, partially or totally, injected into the chips bed, the mixingdevice can be hollow with holes that allow the purge gas to beintroduced into the material by first passing it into the mixing deviceand letting it pass through the holes into the bed of the material beingprocessed. Multiple mixing devices can be connected by a pipe orfunctional equivalent structure.

FIGS. 8 a and 8 b show an embodiment of this mixing device. In detail,the reactor 8-5, with axis of rotation 8-3, has a series of mixingdevices 8-1. These mixing devices are hollow with a large hole, 8-4.Holes 8-4 are for the introduction of the purge gas into the mixingdevices, while holes 8-2, which are smaller than the pellets or chipsbeing processed, are used to distribute the purge gas, 8-6, throughoutthe bed 8-8. The mixing devices 8-1 are linked in series by connectorsdenoted as 8-7. Although believed to be not essential, it is believedthat better efficiency is achieved when the connectors are not incontact with the wall of the reactor. In practice, not in contact meansat least 10 mm from the reactor wall.

Different configurations are possible. For example, one can divide an 80meter reactor into 3 zones: 30 meters, 20 meters, 30 meters, and placetwo mixing devices in each 2 meter section of each zone. The mixingdevices are preferably 180° radially apart from each other. Thedifference between the zones is that the height of the mixing device isvaried, also as a function of the different height of the head of thesolid phase along the axial coordinate of the HCIRR reactor.

In another embodiment, there would be one mixing device per section. Inanother embodiment the reactor is divided into 80 meter sections, withthe first section having one mixing device, the next section having thesecond mixing device with the start of the second mixing device located90° from the end of the first mixing device, the start of the thirdmixing device in the third section located 90° from the end of thesecond mixing device and 180° from the end of the first mixing device.Such a configuration would continue through the zones. All of theseconfigurations have been trailed with better plug flow like behaviourthan having no internals and certainly better than the when the internalmixing device was greater than 1/10^(th) the length of the reactor.

Experimental Results

The first set of experiments were conducted in a glass tube rotatingcircular reactor having an internal diameter of 175 mm, a length of 1800mm, total volume of 43 cubic decimeters and a 1° angle of inclination.

The types of internal mixing devices used are depicted in FIG. 11, withtype 1 (11 a) being a “L” shaped metal piece and type 2 (11 b) being apiece with a 60° bend at the top. The mixing devices were mounted insuch a manner that the equivalent length and actual length were equal,otherwise parallel to the axis of rotation. The mixing devices were each100 mm in length and 30 mm high for the type 1 and 35.43 mm high for thetype 2. At the four 100 mm length axial sections at the end of thereactor (i.e. the end sections at the exit of the chips), four mixingdevices were mounted in the reactor, one for each 100 mm section, 90°apart. More in detail: a mixing device was mounted in the first of the100 mm sections; a further mixing device was mounted in the second andsuccessive 100 mm section, 90° apart from the end of the mixing devicemounted in the first section; a further mixing device was mounted in thethird and successive section, 90° apart from the end of the mixingdevice mounted in the second section and 180° apart from the end of themixing device mounted in the first section; a further mixing device wasmounted in the fourth and successive section, 90° apart from the end ofthe mixing device mounted in the third section, 180° apart from the endof the mixing device mounted in the second section and 270° apart fromthe end of the mixing device mounted in the first section. Experimentswere conducted using chips of commercial bottle grade PET, as well aschips covered of magnetic powder (i.e. ferrite powder) to be injected astracer, in order to determine the distribution curve of the residencetimes at the steady state. Samples were taken at the exit of thereactor, beginning when the tracer was injected and until the end of theexit of the chips with tracer, and the concentration of the chips withtracer or covered by magnetic iron was determined on the samples taken.The number n of the cascade (or series) of the n-CSTR's which might beassimilated to the experimentally obtained residence time distributionwas determined on the basis of the concentration curve of the chips withtracer vs. time and the degree of plug flow was determined accordingly.The results for the examples are in Table I.

TABLE I Rotational Internal Bed Height of Degree of Plug Speed (r.p.m.)Type pellets (mm) Flow (n) 0.9 None 60 60 0.9 None 60 67 0.9 1 60 1160.9 2 60 90 0.9 1 40 60 0.6 None 60 103 0.6 1 60 125 0.6 2 60 75

When mixing devices whose equivalent lengths were greater than about1/10^(th) the total reactor length were mounted in the reactor, some ofthe lifted chips (or granules), quickly traveled, slipping forwardsremaining directly in the plane of the mixing device itself, upward inthe axial direction (with respect to HCIRR reactor axis of rotation),thus giving a detrimental effect on plug flow degree.

On the contrary, when mixing devices whose lengths were less than about1/20 the total reactor length were mounted inside the reactor, the chips(or granules) were lifted by the mixing device and fell down somewherein the middle of the chord of sliding (or of rolling), therefore having50% of the surface sliding path and also 50% of the rough surface motiondisturbance that causes axial dispersion.

We claim:
 1. Use of an inclined cylindrical rotating reactor comprisingan axis of rotation, granules of polyester treated within the reactor, agranules of polyester flow regime, at least two mixing devices and aninert purge gas, for the solid state polymerization of the granules ofpolyester, wherein the granules of material treated within the reactorcomprise a polyester and the reactor has a temperature in the range ofabout 140° C. to about 235° C. the axis of rotation is central and notparallel to the horizontal line perpendicular to the force of gravity,the granules of polyester flow regime is characterized by a FroudeNumber Fr=(ω²×R/g) comprised in the range of 1×10⁻⁴ to 0.5; where ω isthe angular velocity of the reactor; R is the internal radius of thereactor and g is the gravity of acceleration=9.806 m/s; and each of theat least two mixing devices has a height, width, and an equivalentlength defined as the distance between the plane perpendicular to theaxis of rotation that contains the point where the mixing device firstprotrudes from the reactor wall and the plane perpendicular to the axisof rotation that contains the point where the mixing device stopsprotruding from the wall and the equivalent length of the mixing deviceis selected from the group consisting of equivalent lengths less than1/10th the length of the reactor, so as the granules of the polyestertreated within the reactor pass through the reactor due to the force ofgravity as well as the reactor rotation with a plug flow like behaviorsaid use comprising the steps of: introducing the granules into the topof the reactor passing the granules through the reactor while subjectingthe granules to a stream of inert purge gas below the turbulent rate. 2.Use of the reactor of claim 1, wherein there is at least one mixingdevice whose equivalent length is less than 1/20th the length of thehorizontal reactor.
 3. Use of the reactor of claim 1, wherein the atleast two mixing devices are connected in a manner so that the inertpurge gas can pass from the first mixing device to the second mixingdevice through a connector.
 4. Use of the reactor of claim 1, whereinthere is at least one mixing device, whose equivalent length coincideswith the physical length and is less than 1/20^(th) the length of thereactor.
 5. Use of the reactor of claim 1, wherein the axis of rotationis inclined in respect of the horizontal line perpendicular to the forceof gravity of an angle comprised in the range 0.1°-12°.
 6. Use of thereactor of claim 4, wherein the axis of rotation is inclined in respectof the horizontal line perpendicular to the force of gravity of an anglecomprised in the range 0.1°-12°.